Transcription of retinoic acid receptor genes in transgenic mice increases CD8 T-cell subset

Leukocyte homing, fate, and function are controlled by retinoic acid

Although vitamin A was recognized as an "anti-infective vitamin" over 90 years ago, the mechanism of how vitamin A regulates immunity is only beginning to be understood. Early studies which focused on the immune responses in vitamin A-deficient (VAD) animals clearly demonstrated compromised immunity and consequently increased susceptibility to infectious disease. The active form of vitamin A, retinoic acid (RA), has been shown to have a profound impact on the homing and differentiation of leukocytes. Both pharmacological and genetic approaches have been applied to the understanding of how RA regulates the development and differentiation of various immune cell subsets, and how RA influences the development of immunity versus tolerance. These studies clearly show that RA profoundly impacts on cell- and humoral-mediated immunity. In this review, the early findings on the complex relationship between VAD and immunity are discussed as well as vitamin A metabolism and signaling within hematopoietic cells. Particular attention is focused on how RA impacts on T-cell lineage commitment and plasticity in various diseases.

Macrophages engulfing apoptotic thymocytes produce retinoids to promote selection, differentiation, removal and replacement of double positive thymocytes

The thymus provides the microenvironment in which thymocytes develop into mature T-cells, and interactions with thymic stromal cells are thought to provide the necessary signals for thymocyte maturation. Recognition of self-MHC by T-cells is a basic requirement for mature T-cell functions, and those thymocytes that do not recognize or respond too strongly to the peptide-loaded self-MHC molecules found in the thymus undergo apoptosis. As a result, 95% of the thymocytes produced will die and be subsequently cleared by macrophages. This review describes a complex crosstalk between developing thymocytes and engulfing macrophages which is mediated by retinoids produced by engulfing macrophages. The interaction results in the harmonization of the rate of cell death of dying double positive cells with their clearance and replacement, and in promotion of the differentiation of the selected cells in the thymus.

Critical role for all-trans retinoic acid for optimal effector and effector memory CD8 T cell differentiation

A plethora of work implicates important effects of the vitamin A derivative retinoic acid (RA) in myeloid differentiation, whereas fewer studies explore the role of RA in lymphoid cells. Most work on lymphoid cells has focused on the influence of RA on CD4 T cells. Little information about the role of RA in CD8 T cell differentiation is available, and even less on cell-intrinsic effects in the CD8 T cell. This study explores the role of RA in effector and memory differentiation in a cell-intrinsic manner in the context of vaccinia virus infection. We observed the loss of the short-lived effector cell phenotype (reduced KLRG1(+), T-bet(hi), granzyme B(hi)), accompanied by an enhanced memory precursor phenotype at the effector (increased CD127(hi), IL-2(+)) and contraction phases (increased CD127(hi), IL-2(+), eomesodermin(hi)) of the CD8 response in the absence of RA signaling. The lack of RA also increased the proportion of central memory CD8s. Collectively, these results introduce a new role for RA in CD8 T cell activation and differentiation. This new role may have significant implications for optimal vaccine design in which vitamin A supplementation is used to augment effector responses, but it may be to the detriment of the long-term central memory response.

Regulation of CD8(+) T cell functions by RARgamma

Retinoic acid plays a key role in the development and function of the immune system; however, the contribution of each of the three retinoic acid receptors (RARs) to the T cell immune response is not yet well understood. Of these receptors, both RARalpha and RARgamma are expressed in T lymphocytes. While possible functional redundancy thus complicates understanding of the role of each receptor in T cells, emerging data suggest that RARalpha and RARgamma function differently in thymocyte development and that RARgamma is required for both primary and secondary CD8(+) T cell immune responses.

Emerging Role of Combination of All-trans Retinoic Acid and Interferon-gamma as Chemoimmunotherapy in the Management of Human Glioblastoma

Glioblastoma is the most malignant and common type of brain tumor with devastating outcome. Because current treatment modalities are mostly ineffective in controlling and curing glioblastoma, new and innovative therapeutic strategies must be developed. This article describes recent advances in chemoimmunotherapy, which is combination of chemotherapy and immunotherapy, against glioblastoma. We provide an overview of available treatment options for glioblastomas, gaps in our knowledge of immune recognition of these malignant tumors, and chemotherapeutic and immunotherapeutic agents that need to be further explored for designing novel chemoimmunotherapeutic strategy for the management of human glioblastomas. Our recent study demonstrated that combination of the chemotherapeutic agent all-trans retinoic acid (ATRA) and the immunotherapeutic agent interferon-gamma (IFN-gamma) could concurrently induce differentiation, apoptotic death, and immune components in two different human glioblastoma cell lines. We propose that combination of ATRA and IFN-gamma can become an efficacious chemoimmunotherapy for the treatment of human glioblastoma.

Regulation of CD8+ T lymphocyte effector function and macrophage inflammatory cytokine production by retinoic acid receptor gamma

Vitamin A and its derivatives regulate a broad array of immune functions. The effects of these retinoids are mediated through members of retinoic acid receptors (RARs) and retinoid X receptors. However, the role of individual retinoid receptors in the pleiotropic effects of retinoids remains unclear. To dissect the role of these receptors in the immune system, we analyzed immune cell development and function in mice conditionally lacking RARgamma, the third member of the RAR family. We show that RARgamma is dispensable for T and B lymphocyte development, the humoral immune response to a T-dependent Ag and in vitro Th cell differentiation. However, RARgamma-deficient mice had a defective primary and memory CD8(+) T cell response to Listeria monocytogenes infection. Unexpectedly, RARgamma-deficient macrophages exhibited impaired inflammatory cytokine production upon TLR stimulation. These results suggest that under physiological condition, RARgamma is a positive regulator of inflammatory cytokine production.

Inhibition of activation-induced apoptosis of thymocytes by all-trans- and 9-cis-retinoic acid is mediated via retinoic acid receptor alpha

Thymocytes can be induced to undergo apoptotic cell death by activation through the T-cell receptor (TCR). This process requires macromolecular synthesis and has been shown to be inhibited by retinoic acids (RAs). Two groups of nuclear receptors for RAs have been identified: retinoic acid receptors (RARs) and retinoid X receptors (RXRs). All-trans-RA is the high-affinity ligand for RARs, and 9-cis-RA additionally binds to RXRs with high affinity. Because 9-cis-RA is much more potent in inhibiting TCR-mediated death than all-trans-RA, it was suggested that RXRs participate in the process. In the present study various synthetic retinoid analogues were used to address this question further. The results presented suggest that the inhibitory effect of RAs on activation-induced death of thymocytes is mediated via RARalpha, because (1) it can be reproduced by various RARalpha analogues both in vitro and in vivo, (2) the effect of RAs can be inhibited by the addition of an RARalpha antagonist, (3) CD4+CD8+thymocytes, which die on TCR stimulation, express RARalpha. Stimulation of RARgamma, in contrast, enhances the activation-induced death of thymocytes and inhibits its prevention by RARalpha stimulation. RXR co-stimulation suspends this inhibitory effect of RARgamma and permits the preventive function of RARalpha on activation-induced death. Our results suggest a complex interaction between the various isoforms of retinoid receptors and demonstrate that low (physiological) concentrations of all-trans-RA do not affect the activation-induced death of thymocytes because the RARalpha-mediated inhibitory and the RARgamma-mediated enhancing pathways are in balance, whereas if 9-cis-RA is formed, additional stimulation of RXRs permits the inhibitory action of RARalpha.

Downregulation of RARα in Mice by Antisense Transgene Leads to a Compensatory Increase in RARβ and RARγ and Development of Lymphoma

Retinoic acid receptors (RARs) α, β, and γ contain retinoic acid response elements (RAREs) in their promoter regions and respond to their own activation, thus forming an autoregulatory loop. We generated transgenic mice that expressed an antisense construct of the RARα. Homozygous transgenic mice demonstrated 30% to 80% reduction in RARα protein expression in various tissues. Unlike RARα null mice generated by knockout, our antisense mice demonstrated significant compensatory increases in the expression of RARβ and RARγ proteins. Coarse fur, male sterility, and low body weight were other abnormalities observed in these mice. Most importantly, lymphoma developed in 44% of our homozygous transgenic mice at an early stage of life. These data suggest that RARα is necessary for appropriate response of the RARβ and RARγ genes to physiologic changes and deregulation of the RARα in transgenic mice, which resulted in upregulation of RARβ and RARγ, can be associated with lymphomagenesis. Thus, the data support the hypothesis that a balance among the RARs is necessary for appropriate response to various homeostatic needs.